Concealed perimeter surveillance systems employing magnetic anomaly detection are known in the art. Such systems typically comprise a plurality of sensors, typically in the form of buried wire loops, which are connected individually to a central control and display console, which indicates the location of an intrusion.
In order to avoid false alarms due to spurious electromagnetic interference, as from power lines, conventional systems are provided with inhibition apparatus which prevents an alarm indication during the presence of such interference anywhere in the system.